To get it, in the second sigma stage, there's a way to a path completely lined with spikes. And one bat.

You must get the bat to follow you around, repeatedly hitting you so that you can use your invulnerabillity time to walk over, air dash over, or climb though the spikes. Note that this means you have to go just at the right speed - get to far ahead of the bat, and it will fly away. (less info)